Structure of Proteins - Primary to Quaternary - UNSOLVED PRACTICE SET
Chapter: Biomolecules | Topic: Structure of Proteins Primary to Quaternary
STRUCTURE OF PROTEINS - PRIMARY TO QUATERNARY - UNSOLVED PRACTICE SET
Topic: Structure of Proteins Primary to Quaternary
Multiple Choice Questions
Q1. The primary structure of a protein refers to:
- The three-dimensional folding of the protein
- The sequence of amino acids in the polypeptide chain
- The arrangement of multiple polypeptide chains
- The helical or sheet-like arrangement
Q2. The α-helix structure of proteins is stabilized by:
- Disulphide bonds
- Hydrogen bonds between -CO and -NH groups of the same chain
- Ionic bonds
- Hydrophobic interactions
Q3. The β-pleated sheet structure involves hydrogen bonding between:
- Adjacent amino acids in the same chain
- Amino acids far apart in the same chain
- -CO of one chain and -NH of an adjacent chain
- Side chains of amino acids
Q4. Which of the following is NOT involved in stabilizing the tertiary structure of proteins?
- Hydrogen bonds
- Peptide bonds
- Disulphide linkages
- Hydrophobic interactions
Q5. Haemoglobin is an example of:
- Primary structure
- Secondary structure
- Tertiary structure
- Quaternary structure
Q6. Denaturation of a protein involves the disruption of:
- Only the primary structure
- Only secondary and tertiary structures
- Secondary, tertiary, and quaternary structures
- All levels of protein structure
Short Answer Questions
Q7. Distinguish between α-helix and β-pleated sheet structures of proteins.
Q8. What is the denaturation of proteins? Name two agents that can cause denaturation.
Q9. Explain why the primary structure of a protein determines its higher-order structures (secondary, tertiary, quaternary).
Q10. What are disulphide linkages? How do they contribute to the stability of protein structure?
Q11. When you boil an egg, the egg white turns from transparent to opaque white. What change has occurred at the molecular level? Which level(s) of protein structure are affected?
Q12. What is the difference between fibrous proteins and globular proteins? Give one example of each.
Long Answer Questions
Q13. Describe the four levels of protein structure:
(a) Primary structure
(b) Secondary structure (α-helix and β-pleated sheet)
(c) Tertiary structure
(d) Quaternary structure
For each level, explain:
What it is
What forces/interactions stabilize it
One example
Q14. (a) Explain the α-helix structure of proteins with a diagram description. What type of bonds stabilize it? How many amino acids are present per turn of the helix?
(b) Explain the β-pleated sheet structure. Distinguish between parallel and antiparallel β-sheets.
(c) Why is the α-helix more common in fibrous proteins while β-sheets are found in both fibrous and globular proteins?
Q15. (a) What is protein denaturation? Explain the factors that cause denaturation.
(b) A student adds concentrated HCl to egg white albumin and heats it. The protein precipitates. Explain what happens at each level of protein structure.
(c) Why is denaturation usually irreversible? Give an example where denaturation is reversible.
Numerical / Application-Based Problems
Q16. A protein contains 200 amino acid residues.
(a) Calculate the approximate molecular mass of this protein. (Average molecular mass of an amino acid residue = 110 Da)
(b) If this protein forms an α-helix, how many turns of the helix will be present? (Assume 3.6 amino acids per turn)
(c) Calculate the length of the α-helix. (Rise per amino acid residue in α-helix = 1.5 Å)
(d) If the same polypeptide forms a fully extended chain, what would be its approximate length? (Assume each amino acid contributes about 3.5 Å in extended form)
(e) What does this comparison tell you about the compactness of the α-helix structure?
Q17. The following table gives information about different proteins:
| Protein | Number of polypeptide chains | Number of amino acids per chain | Type of structure |
|---|---|---|---|
| Insulin | 2 (A and B) | 21 and 30 | ? |
| Myoglobin | 1 | 153 | ? |
| Haemoglobin | 4 (2α, 2β) | 141 and 146 | ? |
| Keratin | Multiple | Various | ? |
| Ribonuclease | 1 | 124 | ? |
(a) Classify each protein according to its highest level of structure (primary, secondary, tertiary, or quaternary).
(b) Insulin has disulphide bridges between chains A and B, and also within chain A. How many levels of structure does this involve?
(c) Haemoglobin contains four haeme groups, one in each subunit. What is the role of the haeme group? Which metal ion does it contain?
(d) Keratin is found in your hair and nails. Why is it classified as a fibrous protein? What type of secondary structure does it predominantly have?
(e) Ribonuclease can be denatured and then renatured to regain its activity. What does this experiment tell us about the relationship between primary structure and higher-order structures?
Q18. In a school science exhibition, students demonstrate protein structure using paper models.
(a) A student folds a strip of paper into a spiral to represent the α-helix. What does each fold represent? What forces hold the spiral together?
(b) Another student lays strips of paper side by side and connects them with paper clips to represent the β-pleated sheet. What do the paper clips represent? Distinguish between parallel and antiparallel arrangements using this model.
(c) A third student crumples a paper strip into a ball to represent the tertiary structure. What do the creases and folds in the crumpled paper represent? Name four types of interactions that stabilize this structure.
(d) Four students each hold a crumpled paper ball and stand together to represent haemoglobin. What level of structure does this represent? What would happen if one student let go (one subunit dissociated)?
(e) When the paper models are dipped in water (representing denaturation), they unfold. Which levels of structure are destroyed? Can the models be refolded to their original shape? What does this teach us about protein denaturation?